With a continuously growing population, it is becoming increasingly difficult to meet the world's energy needs as well as to kerb greenhouse gas emissions such as carbon dioxide emissions that are considered responsible for global warming phenomena. These concerns have triggered a drive towards more efficient electricity use in an attempt to reduce energy consumption.
One such area of concern is lighting applications, either in domestic or commercial settings. There is a clear trend towards the replacement of traditional incandescent light bulbs, which are notoriously power hungry, with more energy efficient replacements. Indeed, in many jurisdictions the production and retailing of incandescent light bulbs has been outlawed, thus forcing consumers to buy energy-efficient alternatives, e.g. when replacing incandescent light bulbs.
A particular promising alternative is provided by lighting devices including solid state lighting (SSL) elements, which can produce a unit luminous output at a fraction of the energy cost of incandescent light bulbs. An example of such a SSL element is a light emitting diode.
A problem hampering the penetration of the consumer markets by such lighting devices is that it is far from trivial to control the shape of the light output of such devices, at least in a cost-effective manner. This is a particular problem when a highly directional light output is required, e.g. a light bulb having a high degree of collimation or small beam angle, e.g. a beam angle of less than 30°.
Such beam angles can be controlled by the inclusion of collimating lenses into the lighting device. FIG. 1 schematically depicts a cross-section of a prior art collimating lens 10 for collimating the luminous output of a SSL element 20. The collimating lens 10 comprises a central refractive portion 12 surrounded by a total internal reflection prism 14 with the SSL element 20 being centred relative to and opposite the central refractive portion 12.
Such a collimating lens 10 is known to give good collimation results, although a drawback of this lens is that it can become relatively bulky especially when a high degree of collimation of the luminous output of the SSL element 20 is required. This is because the amount of required lens material is inversely related to the beam angle to be achieved. This can be particularly problematic in lighting devices requiring multiple SSL elements to achieve the desired luminous output, as the size of the collimating lens 10 imposes a physical limit on the number of SSL element/collimating lens pairs that can be fitted within the confines of the lighting device. This is particularly relevant when the lighting devices are light bulbs.
In order to address this problem, so-called doughnut lenses have been proposed such as the doughnut lens 30, a cross-section of which is schematically depicted in FIG. 2, which essentially contains the collimating lens 10 in annular form around an aperture 34 of the doughnut lens 30. In other words, the collimating lens 10 is centred on a central axis of symmetry 32. Such a doughnut lens 30 may be used in combination with a circular pattern of SSL elements 20, with each SSL element 20 being centred relative to the central refractive portion 12 of the collimating lens portion 10 as previously explained.
This has the advantage that a single collimating lens, i.e. the doughnut lens 30, can be used in conjunction with a plurality of SSL elements 20, which therefore provides a more compact solution compare to a solution in which each SSL element 20 is provided with a separate (circular) collimating lens 10. However, a large volume occupied by the doughnut lens 30 is occupied by the aperture 34, i.e. the doughnut lens 30 tends to have a relatively large aperture 34, which therefore compromises the collimation performance of this lens.
FIG. 3 schematically depicts a cross-section of another well-known collimating lens 40, which is a Fresnel-type lens comprising a central refractive portion 42 and a plurality of annular prisms 44 around the central refractive portion 42, which annular prisms tend to increase in size in an outward direction (in a direction from the central refractive portion 42 towards the outer edge of the collimating lens 40). As before, the SSL element 20 is centred relative to and opposite the refractive lens portion 42. An example of such a lens is for instance disclosed in U.S. Pat. No. 8,220,975 A1. Such Fresnel-type lenses 40 are more compact than the collimating lenses 10 and allow for a greater area to be used for collimation without increasing the height of the lens. However, the relatively large lateral dimensions of Fresnel-type lenses may make it difficult to form a laterally compact doughnut lens that still achieves sufficient degrees of collimation.